Pili are filamentous structures on the surface of many bacterial cells. Pili are involved in the process of infection by a number of pathogens and also are important to normal bacterial physiology. However, a variety of conditions affect the extent of piliation. In this project, the control of bacterial piliation will be investigated by taking advantage of a recent discovery that synthesis of pilin, the subunit polypeptide of pili, can be induced by dye-sensitized photooxidation. A species of Arthrobacter which is used in these experiments, is not lysed by SDS and thus allows analysis of only surface proteins. Within minutes after cells are exposed to light in the presence of a dye, newly synthesized pilin appears on the cell surface. As the pilin accummulates, an extensive array of pili appears. Synthesis of pilin is measured by quantitative radioautography after resolution of the 21,000-dalton polypeptide by slab gel electrophoresis in the presence of SDS. Induction does not result from the production of singlet oxygen, but apparently results from a radical mechanism of oxidation. Irradiation with near-UV light also results in induction. Experiments will be designed to identify the endogenous sensitizer. The pilin has been purified and work will begin on determining the amino acid sequence of the polypeptide. The experiments are designed to gain information on the mechanisms that control synthesis of pilin and, as a consequence, piliation. Subsequent experiments will investigate the site and mechanism of assembly of pili.